1. Field of the Invention
The present invention relates to a spin valve sensor with an improved antiparallel (AP) pinned layer and more particularly to an AP pinned layer that has reduced current shunting and lower coercivity.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS to cause the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field across the gap between the pole pieces. This field fringes across the gap at the ABS for the purpose of writing the aforementioned magnetic impression in tracks on moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read heads a spin valve sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to an air bearing surface (ABS) of the head and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos xcex8, where xcex8 is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. A spin valve is also know as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer). A pinning layer in a bottom spin valve is typically made of nickel oxide (NiO).
Another type of spin valve sensor is an antiparallel (AP) spin valve sensor. The AP pinned spin valve sensor differs from the simple spin valve sensor, described above, in that the pinned layer of the AP pinned spin valve sensor comprises multiple thin layers, which are collectively referred to as an antiparallel (AP) pinned layer. The AP pinned layer has a ruthenium (Ru) spacer layer sandwiched between first and second ferromagnetic thin layers. The first ferromagnetic thin layer has its magnetic moment oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second ferromagnetic thin layer is immediately adjacent to the free layer and is antiparallel coupled to the first thin layer because of the minimal thickness (in the order of 8 xc3x85) of the spacer layer between the first and second ferromagnetic thin layers. The magnetic moment of the second ferromagnetic thin layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first ferromagnetic layer.
The AP pinned layer is preferred over the single layer pinned layer. The magnetic moments of the first and second layers of the AP pinned layer subtractively combine to provide a net pinning moment of the AP pinned layer. The direction of the net moment is determined by the thicker of the first and second thin layers. The thicknesses of the first and second thin layers are chosen to reduce the net moment. A reduced net moment equates to a reduced demagnetization (demag) field from the AP pinned layer. Since the antiferromagnetic exchange coupling is inversely proportional to the net pinning moment, this increases exchange coupling between the first ferromagnetic film of the AP pinned layer and the pinning layer. The high exchange coupling promotes higher stability of the head. When the head encounters elevated thermal conditions caused by electrostatic discharge (ESD) from an object or person, or by contacting an asperity on a magnetic disk, the blocking temperature (temperature at which magnetic spins of the layer can be easily moved by an applied magnetic field) of the antiferromagnetic layer can be exceeded, resulting in disorientation of its magnetic spins. The magnetic moment of the pinned layer is then no longer pinned in the desired direction. A reduced demag field also reduces the demag field imposed on the free layer which promotes a symmetry of the read signal. The AP pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein.
The first and second ferromagnetic layers of the AP pinned spin valve sensor are typically made of cobalt (Co). Unfortunately, cobalt has high coercivity, high magnetostriction and low resistance. When the first and second ferromagnetic layers are formed they are sputtered deposited in the presence of a magnetic field that is oriented perpendicular to the ABS which sets the easy axis (e.a.) of the ferromagnetic films perpendicular to the ABS. During operation of the head the AP pinned layer is subjected to extraneous magnetic fields that have components parallel to the ABS, such as components of the write field. These extraneous fields, combined with heating of the pinning layer, can cause the pinning layer to lose its pinning strength (exchange coupling) and allow the magnetic moments of the ferromagnetic layers to switch from being perpendicular to the ABS to some other direction. If the coercivity of the ferromagnetic films is higher than the exchange field that urges the magnetic moments of the ferromagnetic layers back to their original positions the magnetic moments of the ferromagnetic layers will remain in the wrong direction. This renders the read head inoperable.
Cobalt (Co) has a high negative magnetostriction. The negative sign determines the direction of any stress induced anisotropy. When a magnetic head is lapped, which is a grinding process, nonuniform compressive stresses occur in the layers of the sensor. Because of the magnetostriction and the stresses the cobalt (Co) ferromagnetic films acquire a stress induced anisotropy that is parallel to the ABS. This is the wrong direction. The stress induced anisotropy may rotate the magnetic moment of the first and second ferromagnetic layers of the AP pinned layer to some extent from perpendicular to the ABS in spite of the exchange coupling field tending to maintain the perpendicular position. This condition can cause read signal asymmetry.
The low resistance of the cobalt (Co) ferromagnetic films of the AP pinned layer causes a portion of the sense current to be shunted past the free and spacer layers. This causes a loss of read signal.
Efforts continue to increase the spin valve effect of GMR heads. An increase in the spin valve effect equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head. Promoting read signal symmetry is also a consideration. This is accomplished by reducing the magnetic influences on the free layer. A search still continues to lower the coercivity, substantially eliminate magnetostriction and increase the resistance of some of the critical layers of the spin valve sensor.
The present invention provides a material for the pinning layer that has higher resistivity and lower coercivity than the cobalt (Co) material typically employed in the pinning layer. This material is selected from the group comprising cobalt iron niobium hafnium (CoFeNbHf), cobalt iron niobium (CoFeNb), cobalt iron hafnium (CoFeHf) and cobalt niobium hafnium (CoNbHf) wherein the preferred atomic weight percentage of CoNbHf is 87/11/2. The preferred material is cobalt iron niobium hafnium (CoFeNbHf). While cobalt (Co) has a resistivity of 10-12 ohms cm, cobalt iron niobium hafnium (CoFeNbHf), has a resistivity of 110 ohms cm. Further, while cobalt (Co) has a coercivity of 50-200 Oe, cobalt iron niobium hafnium (CoFeNbHf) has a coercivity of 5-10 Oe wherein the atomic weight percentages were 86.5/0.5/11/2.
As mentioned hereinabove, the AP pinned layer has first and second ferromagnetic layers separate by a very thin ruthenium (Ru) layer. The first ferromagnetic layer is exchange coupled to the pinning layer with its magnetic moment oriented in a first direction and the second ferromagnetic layer is exchange coupled to the first ferromagnetic layer with its magnetic moment oriented in a second direction antiparallel to the first direction. In a preferred embodiment the first ferromagnetic layer is cobalt iron niobium hafnium (CoFeNbHf) and the second ferromagnetic layer is cobalt (Co). With this arrangement the first ferromagnetic layer will reduce current shunting and have a lower coercivity to stabilize pinning of the pinned layer. Cobalt (Co) is a preferred material for the second ferromagnetic layer since it enhances the spin valve effect by being adjacent to the spacer layer. In some arrangements, however, it may be desirable for the second ferromagnetic layer to be cobalt iron niobium hafnium (CoFeNbHf).
In still other embodiments of the invention one or both of the first and second ferromagnetic layers may have first and second films where one of the films is cobalt (Co) and the other film is cobalt iron niobium hafnium (CoFeNbHf). The invention is applicable to top or bottom spin valve sensors. In a top spin valve sensor the pinned layer is pinned by a pinning layer at the top of the sensor (pinning layer is closer to the write head than the pinned layer) and in a bottom spin valve sensor the pinned layer is pinned by a pinning layer that is at the bottom of the sensor pinning layer is (further away from the write head than the pinned layer). In a bottom spin valve sensor nickel oxide (NiO) is typically employed for the pinning layer. In this type of sensor a nickel iron (NiFe) interface layer is employed between the pinning layer and the pinned layer for the purpose of promoting exchange coupling. Still further, in some embodiments of the invention a spin valve enhancement layer is employed. The spin valve enhancement layer is a very thin layer of cobalt (Co), such as 10 xc3x85, that is located between and interfaces each of the spacer layer and the pinned layer. The invention can also be employed for simple spin valve sensors where a single pinned layer is employed.
An object of the present invention is to provide material for a pinned layer for a spin valve sensor that has higher resistivity and lower coercivity than prior art materials employed for pinned layers.
Another object is to provide a spin valve sensor that has improved pinned layer stability in the presence of extraneous fields.
A further object is to provide an AP pinned spin valve sensor wherein a first ferromagnetic layer antiparallel coupled to the pinning layer has high resistivity and low coercivity and a second ferromagnetic layer interfacing the spacer layer is cobalt (Co) for promoting a GMR effect.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.